Atrial fibrillation is a condition that results from abnormal electrical activity within the heart. This abnormal electrical activity may originate from various focal centers of the heart, and the electrical activity generally decreases the efficiency with which the heart pumps blood. It is believed that some of the focal centers reside in the pulmonary veins of the left atrium. It is further believed that atrial fibrillation can be reduced or controlled by structurally altering or ablating the tissue at or near the focal centers of the abnormal electrical activity to form a “conduction block.”
One method of structurally altering tissue of the heart and pulmonary veins is to make, for example during open-heart surgery, a series of incisions in a maze-like pattern in the atria, and sew the incisions back together. As the incisions heal, scar tissue forms, and the scar tissue may block the conductive pathways thought to cause atrial fibrillation. The procedure, which was developed under the direction of Dr. James Cox and refined over a period of years, may be referred to as a “maze” procedure, a “Cox maze” procedure, a “Cox maze III” procedure; or the procedure may be referred to by various other names.
A less invasive method of structurally altering tissue of the heart and pulmonary veins involves ablating tissue through the use of an ablation catheter. One example type of ablation catheter delivers radio frequency (RF) energy to ablate tissue; another example ablation catheter ablates tissue with a heat source; another example ablation catheter delivers cryotherapy to ablate tissue by freezing it.
Cryotherapy may be delivered to an appropriate treatment site inside a patient's heart or circulatory system with a cryotherapy catheter. A cryotherapy catheter generally includes a treatment member at its distal end, such as an inflatable balloon having a cooling chamber inside. To deliver the cryotherapy, the inflatable balloon may be introduced at a treatment site inside a patient, and the balloon may be positioned and inflated. Once the balloon is positioned, a cryogenic fluid may be provided by a source external to the patient at the proximal end of the cryotherapy catheter, and delivered distally through a lumen to the cooling chamber, where it may be released. Release of the cryogenic fluid into the chamber can cool the chamber (e.g., through the Joule-Thomson effect), and correspondingly, the balloon's outer surface, which may be in contact with tissue that is to be ablated. Gas resulting from release of the cryogenic fluid may be exhausted proximally through an exhaust lumen to a reservoir or pump external to the patient. As a result of the release of the cryogenic fluid into the chamber and the exhausting of the resulting gas from the chamber, tissue adjacent to the balloon may be cooled to a therapeutic level (e.g., 0° C., −20° C., −60° C., −80° C., or some other appropriate value) for an appropriate period of time.